gh-99300: Replace Py_INCREF() with Py_NewRef() in _elementtree.c

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• Replace Py_INCREF() and Py_XINCREF() using a cast with Py_NewRef() and Py_XNewRef() in Modules/_elementtree.c.
• Make reference counting more explicit: don't steal implicitly a reference on PyList_SET_ITEM(), use Py_NewRef() instead.
• Replace PyModule_AddObject() with PyModule_AddObjectRef().

• Issue: Replace Py_INCREF()/Py_XINCREF() usage with Py_NewRef()/Py_XNewRef() #99300